Microbially Influenced Corrosion And Filter Plugging - Don't You Wish They Were Easy To Diagnose?

ABSTRACT

Root cause analysis is the process used to identify the fundamental cause for an undesirable condition. Premature filter failure due either to plugging or other mechanism is generally perceived to be an acute problem. Despite the waxing and waning of their symptoms, systemic microbial infections are chronic. Understanding the nature of microbially influenced corrosion and microbially mediated filter failure can facilitate better system monitoring and maintenance processes and system design.

This presentation reviews the fundamental mechanisms of microbially influenced corrosion and premature filter failure. It also provides guidelines for diagnosing microbially mediated problems in petroleum product systems ranging from crude oil production to waster-based metalworking fluids.

INTRODUCTION

Throughout the petroleum industry, cost-of-quality discussions are frequently conflictual and left unresolved. Though generally perceived as acute, filter plugging is really a chronic cost-of-quality problem. Moreover, filter-plugging is sometimes indicative of another chronic problem: microbially influenced corrosion or MIC.

The causes of premature filter plugging include many conditions that have no apparent link to biological activity in petroleum systems. However, as I'll discuss during this presentation, microbes in remote parts of a petroleum system play critical, unrecognized roles in the symphony of events that lead to premature filter failure.

Historically, MIC was the acronym for microbially induced corrosion. By the early 1980's, researchers realized that microbial communities often played secondary or lowerorder roles in corrosion processes. Conveniently, the acronym could be preserved, while it's meaning changed rather substantially.

Today, I shall offer some working definitions for both filter failure and MIC. I'll then suggest an approach for diagnosing microbially influenced filter failure and corrosion. I shall use the term petroleum product(s) throughout this presentation, since the general principals I'll be discussing in this presentation are equally applicable to crude oil, liquid fuels, lubricants and water-based metalworking fluids.

MICROBIALLY INFLUENCED CORROSION

As I implied during my introductory comments, our understanding of MIC has changed considerably since von Wolzogen Kuhr and van der Vlugt proposed an anaerobic corrosion model in 1934 [1]. As with much of our understanding of ecological processes, our current MIC model is considerably more complex than the one originally proposed. Figure 1 provides a schematic model of the various roles of microbes in ferrous metal corrosion.

MIC may be physical, chemical or a combination thereof. As soon as pioneering microbes attach to metal surfaces, electropotential (galvanic) gradients form. These are driven by the surface REDOX potential differences between the exposed and covered surfaces. Within 24 to 48 hours, biofilm microcolonies have matured sufficiently so that the physicochemical conditions within the biofilm are substantially different from those of the surrounding fluid. Although the bulk petroleum product may contain water in the parts per million range, biofilms are mostly water.

Starkey [2] and other early investigators believed that the sulfate reducing bacteria (SRB) were the primary bacteria involved in MIC. We now understand that SRB participation in the process was overestimated. Aerobes (bacteria and fungi that require oxygen) and facultative anaerobes (bacteria that can thrive whether or not oxygen is available) generate organic acid wastes. Although these are weak acids, their protons can react with chloride, nitrate and sulfate ions to form hydrochloric, nitric and sulfuric acid, respectively. In contrast to the weak organic acid metabolites, these strong acids can attach metals aggressively. Microbially generated acids also partition into petroleum products, thereby increasing product corrosivity. Some biodeteriogenic microbes routinely recovered from petroleum product systems depend on the petroleum product as their food. Hydrocarbon and nonhydrocarbon constituents provide all of the nutrients these microbes need to thrive in these systems. Other microbes (for example the SRB mentioned above) depend on organic chemicals that the hydrocarbon degraders secrete as wastes (metabolites).

Typically, microbes don't work as pure cultures (one type of microbea) in industrial systems. Instead, consortia form within biofilms. Several different types of microbes work together to mediate changes that none of the individuals could. One example is the manner in which facultative anaerobes create an environment suitable for SRB growth. The facultative anaerobes scavenge oxygen and convert complex organic molecules in to simpler ones that the SRB can eat. Without the facultative anaerobes, the SRB wouldn't have a suitable habitat. Recent research at the University of Montanab and elsewhere has demonstrated that biofilm ecology is much more complex than microbiologists had originally thought. Cells within the biofilm differentiate. Similar to what happens in higher organismsc, cells from individual types of microbes take on very different characteristics depending on their position within the biofilm. Not only are different microbes working in concert, but also individual types of microbes are behaving as though they were genetically different types of microbes.

The biofilm's gross structure is also quite complex. Although they appear to be just amorphous masses of slime, biofilms are highly structured, with pores and channels for transporting nutrients and eliminating toxic metabolites. In many respects, a mature biofilm resembles a simple multicellular organism. Small wonder that we are barely scratching the surface in terms of understanding MIC.

For our current discussion, the critical issue is that ferrous and ferric hydroxide and ferrous sulfide are insoluble in either petroleum products or water. Thus iron particulates abraded from MIC sites are transported to filters where they may be the dominant retained material. It's not just a matter of SRB. Nor it is a simple question of recovering microbes from bulk fluid samples. Since the microbes are predominantly localized within biofilms, significant infections may go unnoticed until after systems fail.